Dental materials mixing apparatus



Sept. 19, 1967 M. w. BOLDE 3,342,464

DENTAL MATERIALS MIXING APPARATUS Filed Oct. 25. 1965 2 Sheets-Sheet 1 INVEN'I'OR. M/CHflEL W. 01.05

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pTTOR VE y Sept. 19, 1967 M. w. BOLDE 3,342,460

DENTAL MATERIALS MIXING APPARATUS Filed Oct. 23. 1965 2 SheetsSheet :3

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azzam/z 14 TTOQME y United States Patent 3,342,460 DENTAL MATERIALS MIXING APPARATUS Michael W. Bolde, Los Angeles County, Calif. (8824 Monogram St, Sepulveda, Calif. 91343) Filed Oct. 23, 1965, Ser. No. 503,981 Claims. (Cl. 259-122) This invention relates to dental material mixing apparatus and more particularly to the configuration and relationship of a rotary impeller blade incorporated into the apparatus and adapted to be moved within and along the inner surface of a bowl or similar vessel in such a manner as to provide a thorough and complete mixing movement or circulation of the substance of material while tending to remove or prevent entrapment of air therewithin.

Apparatus has been employed in the past for removing or purging air from substances, materials and compositions that are being mixed in a bowl or container. Such means are known which employ various vacuum techniques as well as employing various means to apply a differential pressure in order to force air from the substance being mixed. Generally problems have been encountered when employing such conventional means which stern largely from the fact that the utilization of vacuum techniques requires expensive and complex equipment which while employing differential pressure techniques still permits the presence of undesirable free air pockets, air bubbles, entrained air or the like in the substance. This problem is particularly acute in the dental field during the preparation of various materials employed in the making of dental impressions and models in which the various powders are mixed with water. To construct dental impressions, it is customary in the field to mix a dental preparation known in the art as alginate and also to mix a preparation known in the art as stone.

The presence of free air, entrapped air or dissolved air in the dental materials not only leads to contamination of the substance per se but contributes to an uneven consistency of mixture so that the material cannot be readily applied uniformly and, after the substance or material has been cured, cracks and/or shrinkage occur which distort or otherwise render the substance unsuitable for the construction of dental impressions and models.

Alginate is a rubbery material currently in use and is formed by mixing a dry powder of the alginate material with water. As soon as mixture occurs, the alginatebecomes rubbery and begins to set up -by which is meant that the material coheres to itself so that, if it were mixed in a bowl and permitted to remain in the bowl after mixing, it would result in a rubbery mass having the molded configuration of the interior of the bowl.Although the mass would be deformable, it would not be capable of acquiring any other configuration. In use, as soon as the material is mixed and has suflicient cohesion to be handled, it is placed in the patients mouth at the desired locations on the teeth and/ or gums and permitted to then cure or set in situ so that, after such setting, it can be removed from the patients month by merely peelingly lifting it therefrom. It is still rubbery, of course, but it has now set in the shape of the molded form of the teeth and/or gums and, although flexible and someparticles absorb as much water as what resilient, it is now usable as a rubber-like mold for casting a dental model of the teeth and/or gums. The common and standard method of mixing alginate is to manually spatulate the powder and water in a deformable and flexible bowl which normally takes a considerable amount of time. Great difliculty is encountered because the powdered particles that have become mixed with the water during initial spatulation start to set while other powder particles may not have yet come into contact with water in the mixing spatulation; thus, the dental technician or dentist experiences progressively increasing resistance during spatulation of the material in an effort to assure complete and uniform mixing. Also, the patients jaws are normally completely immobilized just prior to commencement of the mixing operation so that his mouth is prepared and ready for application of the alginate. In the case of children, in particular, this is a most diflicult and trying experience and often results in the child having moved impatiently or otherwise suffering from seemingly unbearable discomfort by thetime the alginate mixture is ready and often rejecting the application of the alginate because of the long delay, as a result of which the procedure has to be repeated with ensuing and additional inconvenience and expense. Still further, once the alginate has been mixed in the bowl, it becomes too diificult to continue spatulation and must be removed from the bowl and placed in the patients mouth, after which it normally requires at least three minutes or longer to become set, during which time the patient must remain uncomfortably immobile.

With respect to the stone, it normally is of the type commonly known as plaster of Paris or similar materials which present problems similar to that of mixing alginate but even more critical in some aspects. The stone commences in a powdered form and is mixed with water in the flexible bowl by manual spatulation or by one of a variety of vacuum processes. It begins to cure or set when mixed and, rather than being rubbery like the alginate, becomes completely hard and rigid after it has completely set. Unlike the alginate, which does not seemingly have much problem in the way of air bubbles, air entrapment is a major and critical problem with stone; namely, entrapped air in the stone mixture will cause shrinkage and/or cracking of the model after it has set up in the alginate mold. Another problem which occurs is that the materials used often release gaseous vapors when combined with water so that it is not merely a.

matter of purging air that might normal mixing operations but also purging the gas that is released, liberated, generated and so forth by the material itself during chemical combination with water. Furthermore, it is critical that all of the powder be thoroughly mixed with the water so that the powder is possible since the presence of non-hydrated powder in the ultimate mixture causes non-uniformity and structural. defects and failure in the final set product.

In addition, many dental mixing bowls are be entrapped during circular in blades be mounted on an arm extending perpendicularly from a central rotating vertical post or column. The problem with this arrangement is that it is extremely difficult to ensure that the blade mounting post is in proper alignment with a motorized power source so that, upon the application of power to the rotary blade post, the mixing edge portion of the blade will have proper clearance between the inner wall of the bowl so that neither the mixer, driving shaft or the wall of the bowl will be damaged. Misalignment of the drive shaft with the power source may also cause breakage of the impeller blade due to any eccentricities or irregularities in the generally circular shape of the mixing bowl or, in the event that damage does not occur, an uneven scraping as well as irregular mixing force will result which also adversely affects the removal of air from the mixing substance.

Accordingly, the apparatus of the present invention avoids the problems and difficulties encountered with prior mixing apparatus for dental compounds by providing a rotary impeller means which, when assembled with a bowl holding the substance to be mixed, can be inserted into an electrical mixer whereby the insertion of the blade into the mixer effects the energization of the motor to cause powered rotation of the blade. A feature of the invention resides in the fact that powered rotation cannot be initiated unless the drive shaft for the blade is properly located with respect to the power mixer so that no damage will occur to the mixing bowl, the rotary blade or to the mixer itself. The invention contemplates specially configured impeller blades which permit the proper, efiicient and rapid mixing of dental compounds such as alginate and stone for removing the presence of air therefrom without the employment of vacuum equipment or techniques. In this connection, various modified lids for the bowl are employed in combination with the rotary mixing blade to effect the proper mixing of the dental substance wherein a particular bowl lid is employed for the mixing of alginate and another lid is employed for the mixing of stone.

The present invention accomplishes the result of purging air from the mixture, in general, by providing a blade rotatable about an axis along its central mounting post which lies on the vertical central axis of the bowl containing the substance. A scraper or mixing portion formed along the edge marginal regions of the blade extends from the pivot rotary mount carried by a bowl lid or cap to the bowl inner surface. The blade is rotatable on a post which is detachably connected to a rectilinearly movable rotary motor drive which effects powered blade rotation. The area of the scraper portion which moves forward into the substance contained in the bowl is in such relation thereto that the substance to be mixed is driven downwardly against the curved wall of the bowl. The forward face of the blade is therefore maintained in an angualrly downward position with respect to the direction of rotation while the blade post is substantially coaxial with the direction of rotational movement of the blade. A lid is employed which fits snugly over the opening into the bowl. When mixing alginate, the bottom of the lid lies in a plane immediately adjacent the top of the impeller blade while the bottom of the lid employed for mixing stone provides a substantial gap or clearance between its undersurface and the top of the impeller blade. With the present invention, it is therefore possible to remove the presence of free air, entrained air, entrapped air or the like from material being mixed and yet avoid the possibility of damage to the blade.

Therefore, it is an object of the present invention to provide a novel dental mixing apparatus for removing or purging the presence of air from dental material being mixed which employs mechanical manipulation of the materials without the necessity for employing vacuum techniques.

Another object of the present invention is to provide a novel dental mixing apparatus in which a powered impeller blade is employed in such a fashion that the driving power thereto cannot be applied unless the impeller blade is in a proper and safe position prior to the energization of the motor.

Another object of the present invention is to provide a novel dental apparatus for mixing dental compositions such as alginate and stone whereby the impeller blade is rotated while oriented in a direction such as to drive the composition or materials to be mixed downwardly against the inner wall of the container or bowl holding the materials.

Still another object of the present invention is to provide a novel dental mixing apparatus for removing the air from such dental compounds as alginate and stone which will readily accommodate interchangeable caps or lids for the bowl Containing the material to be mixed depending upon whether the dental composition relates to alginate or stone.

Still another object of the present invention is to provide a novel dental material mixing apparatus including a rotary blade which is adapted to thoroughly mix dental compounds while such compounds are progressively setting and which is capable of eflfecting the removal of air from the material being mixed.

Still a further object of the present invention is to provide a novel dental mixing apparatus having relatively long life and requiring little or no maintenance and which is readily operable by a dental technician in the typical surroundings of a dental office.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings, in which:

FIGURE 1 is a side elevational view, partly in section and partially exploded, of the dental materials mixing apparatus of the present invention illustrating the mixing bowl, lid and rotary blade for conditioning alginate detached from the mixer as indicated in solid lines and attached to the mixer as indicated in broken lines;

FIGURE 2 is an enlarged fragmentary view, partly broken away and sectioned, of the electrical switch means incorporated into the mixer of FIGURE 1 for initiating and terminating operation of the power mixer;

FIGURE 3 is a bottom plan view of the rotary mixer blade enclosed by the bowl as taken in the direction of arrows 33 of FIGURE 1;

FIGURE 4 is an enlarged fragmentary sectional View of the rotary mixing blade and lid as taken in the direction of arrows 4-4 of FIGURE 3;

FIGURE 5 is a sectional view of a mixing bowl and rotary mixing blade therefor illustrating a modified lid or cap suitable in connection with the mixing of dental materials such as stone;

FIGURE 6 is a sectional view of a mixing bowl incorporating a modified rotary mixer blade and lid therefor for effecting the mixing or conditioning of dental materials such as alginate;

FIGURE 7 is a perspective view of the rotary blade illustrated in the embodiment of FIGURE 6;

FIGURE 8 is an elevational view of the rotary mixing blade illustrated in FIGURE 6; and

FIGURE 9 is a bottom end view of the blade shown in 7 FIGURE 8 taken in the direction of arrows 99.

Referring to the drawings in detail, there is seen a dental mixing apparatus in accordance with the present invention which is suitable for mixing such dental materials as alignate or stone. The apparatus as illustrated in FIGURE 1 includes a motor portion indicated in the direction of arrow 11 and a mixing portion indicated in the general direction of arrow 12. The motor portion includes a housing 13 which is suitable for removable mounting onto the surface exterior of a wall 14 by means of a bracket 15 and serves to house a conventional motor having a set of motor windings 15 surrounding a rotatably mounted armature 16 which is suitably carried on a shaft 17 rotatably journaled in end bearing mounts 18 and 20 respectively. The end of shaft 17 projecting from mount 20 carries a chuck member 21 having a receptacle 22 formed therein of square or hexagonal configuration which is adapted to detachably receive a rotary blade shaft 23 therein. The blade shaft 23 includes on its upward end a series of flat sides 24 which conforms to the shaped receptacle 22 provided in the chuck member 21 so as to effectively impart the rotary motion of the motor shaft 17 to the blade shaft 23.

The upper end of shaft 17 is in coaxial alignment with a pushbutton 25, as shown in FIGURE 2, which forms a part of a microswitch device 26 suitably mounted on a bracket 27 secured to the motor housing 13. The button 25 is arranged for rectilinear movement into and out of the switch housing 26 so as to make and break electrical circuit contact between the motor energizing lead wires 28 and 30. A spring 31 is provided within a button cylindrical housing 32 for normally biasing the button out of contact with the switch mechanism to open the electrical circuit between wires 28 and 30 so that the motor is deenergized. However, upon upward movement of the shaft 17, the pushbutton is urged against the spring bias of spring 31 to make connection between the wires 28 and 30 within the switch 26 to energize the motor and, therefore, to cause rotation of shaft 17.

The upward urging of shaft 17 is achieved by inserting the blade shaft or mounting post 23 into the chuck member 21. A feature of the present invention resides in the fact that the material to be mixed is contained within a flexible bowl 33 which is known in the trade by the trademark Flexibole so that the bottom of the bowl can be hand-held and manually urged upwardlyto move the end of shaft 17 into operable contact with tne switch 26 via the button 25. As shown in broken lines in FIGURE 1, the shaft 23 is in operable contact with the chuck member 21 and the end of shaft 17 is pressing against the bias of pushbutton 25 so that the motor is energized. However, as shown in solid lines, the bowl 33 and and the shaft 23 are detachably connected from the chuck member 21 so that the end of shaft 17 is disconnected or out of operable contact with the pushbutton 25 as is illustrated in FIGURE 2 so that the motor is de-energized.

Continuing with the description of the mixing portion 12 as shown in FIGURE 1, the flexible bowl 33, which may be composed of plastic or any other suitable material, is shown having an interior wall surface 34 in which the material to be mixed is held. The blade mounting shaft 23 extends vertically down into the center of the bowl from the motor mounted on the housing 13 and through the axial center of a cover or lid indicated generally at 35 which seals the upper portion of the bowl. The lid or cap member 35 is preferably made of porcelainized aluminum and includes a top plate 36 having a continuous circular edge marginal region which extends beyond the periphery of the opening to the bowl 33 and including a downwardly depending body portion 37 provided with an annular peripheral sealing surface 38 which seats within the opening into the bowl against the interior wall surface 34. It is to be noted that the peripheral sealing surface 38 is substantially conically tapered so as to more etfectively mate with the contour of the wall surface 34 to effect efiicient sealing.

1 An impeller is shown mounted on the post 23 which includes a'pair of flat blades 40 and 41 arranged on opposite sides of the post 23 as more clearly shown in FIG- URE 3. The impeller blades, in combination with the lid 35 shown in FIGURE 1, are particularly suited for the mixing of alginate. It is to be noted that each of the blades 40 and 41 includes an aperture 42 located in the upper portion of each blade and an aperture 43 of reduced diameter located generally beneath the aperture 42. The apertures serve to adequately permit the passage of alginate or other materials being worked upon to pass therethrough to assist in the mixing of the material as well as to free the material from captured air as the blades rotate in a clockwise direction (asseenin FIGURE 3). To insure that the mixture or composition is throughly mixed, an elongated projection 44 is provided at the extreme bottom end of the shaft 23 and is located in close proximity to the bottom of the bowl inner wall surface 34. By providing such a projection, material located in this area is thoroughly mixed which otherwise might form a dead spot and not become subject to mixing. Also, it is to be noted that the overall configuration of the edge marginal regions of the blades 40 and 41 conform to the contour of the inner Wall surface 34 and are maintained in close proximity thereto to effectively remove or agitate material directly adjacent the interior wall surface. I have found that by angling the blades 40 and 41 within an approximate range of 12-16", an inclined planar face 45 is provided on each blade which serves to drive the material downwardly towards the bottom of the bowl as the blades are rotated in the aforesaid clockwise direction.

With respect to FIGURE 4, an enlarged view of the impeller blade mounting means with the lid 35 is shown which includes upper and lower bearing members 46 and 47 which are press-fit into a bore 48 provided in the center of the lid 35 and preferably are composed of a plastic material such as nylon, for example. The bore is coaxial with the bowl 33 and, therefore, the shaft 23 is coaxial therewith also. It is to be noted that the shaft 23 is journaled through the bearing members 46 and 47 and that one end of the shaft 23 is threadably engaged at 49 with a post element 50 which is coextensive with the impeller blades 40 and 41 and suitably attached thereto, such as by welding, for example.

It is to be particularly noted that the upper edge of the impeller is in close proximity to the bottom or underside of the lid 35 so that, in combination with the closeness of the blade edge marginal region with the interior wall surface of the bowl, thorough mixing and agitation of material stored in the bowl between the bottom of the lid 35 and the interior wall surface 34 is assured and the material is completely subjected to the mixing action effected by the impeller. It is desirable that the lid embodiment 35 as shown in FIGURE 1 be employed for the mixing of alginate which has a lesser density than that of stone. By such an arrangement, the stirring of alginate can continue in the bowl even after thorough mixing has occurred so that the alginate begins to set up while still in the bowl; thus, the alginate need be placed in a patients mouth for the purpose of making impression only a relatively short time, e.g., thirty seconds, as compared to normally three minutes or longer when conventional dental mixing equipment is used.

However, when it is desired to mix adental composi-' tion such as stone which may be formed from mixing plaster of Paris with water, a lid 51 configured as shown in the embodiment of FIGURE 5 has been found desirable. The general configuration of the lid 51 is identical to the lid 35 used for mixing alginate with the exception that the thickness of the lid 51 is substantially reduced so as to provide a substantial gap or clearance which preferablyis about 0.4 inch between the bottom of'the lid and the top edge 52 of the impeller blades 40 and 41. Such-an arrangement permits a certain portion of the mix to pass not only through the passages 42 and 43 of the impeller-blades but also over the top edge 52 of the blades as the impeller is rotated. Generally when employing conventional dental material mixing blades, the mixing material is driven upwardly along the inclinedwall of the bowl and is forced between the bowl edge and the lid so that irregular and non-uniform mixing occurs. Also, the mix spills over to the outside of the bowl which necessitates added inconvenience and handling. Obviously, this also reduces the effectiveness of the blades to purge the mix of entrapped air.

. The bowl 33 as shownin FIGURE 5 is provided with a liner 53 which preferably is composed of a relatively rigid plastic material and is adapted to cover the interior wall of the bowl 33. The employment of the liner 53 permits the mixing of one preparation to be done within the liner followed by the removal thereof so that another mixing procedure can take place in the bowl 33 immediately thereafter. This arrangement has been found to be very advantageous since the bowl need not be cleaned prior to the second mixing procedure, thus saving time.

Referring now to FIGURES 69, a lid 54 and impeller 55 are shown which represent a preferred embodiment for mixing alginate. In this embodiment, the lid 54 includes a hollow body portion having a tapered annular outer surface 56 which seats in sealing relationship with the inner wall surface of the cup or bowl 33. To insure proper seating, the lid is provided with an annular edge 57 which fits over the extreme edge of the opening to the interior of the bowl 33. Rotatably mounted in a nylon bearing 58 which is force fitted into a bore in the center of the lid 54, is a drive shaft 59 having a series of fiat surfaces 60 on one end thereof for effecting operable connected with the chuck member carried by the mixing unit. The shaft 59 and impeller 55 are maintained in position by means of an adjustable collar 61 which is provided with a set screw 62 for securing the collar 61 to the shaft 59. The lower end of the shaft 59 is provided with the impeller 55 that includes a pair of flat coaxial sections 63 and 64, as seen more clearly in FIGURE 7, wherein the section 63 is received into a slot 65 provided in the end of shaft 59, after which the impeller is secured thereto as by heliarc welding. It is to be particularly noted that the sections 63 and 64, as seen in FIGURE 9 are disposed at almost a right angle with respect to one another so that the blade segments 66 and 67, integrally formed with the sections 63 and 64, present angularly downwardly disposed forward faces 68 and 69 that are employed to forcibly urge the alginate material in a downward direction as the blade is rotated in a clockwise direction (as viewed in FIG- URE 9). The impeller blades are provided with a curved edge which matches the curved contour of the inner sur face of the bowl, and each presents a substantially planar configuration with such planes being disposed at approximately a 30 angle with respect to each other, symmetrical about the vertical axis. It can thus be seen that the entire impeller assembly is rotatable by the end of the shaft 59 and coaxially therewith.

It is to be noted that the width of the blade face is substantially constant throughout and the blade forms a substantially continuous parabolic shape as illustrated in FIGURE 6.

As shown in the drawings, the present invention utilizes impeller blades in which the forward face of the blade (the face moving forward into the material in the bowl) forms an angle with the bowl wall of approximately In this manner, the blade effectively mixes the dental materials so that the presence of air is greatly reduced, if not entirely removed. The rotary movement of the blade produces a circulating and mixing motion in the material in the vessel. It is to be understood that, when mixing stone, the impeller means of FIGURES 6-9 is employed with a lid providing a gap or clearance with the top edge of the blades, such as lid 51 of FIGURE 5, so that the stone mixture which may be impelled upwardly against the lid is permitted to descend therefrom by gravitational forces rather than being impelled outwardly.

While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

I claim:

1. Apparatus for stirring and mixing dental materials such as. stone in a flexible bowl for holding the materials comprising:

a high speed impeller coaxially disposed within the bowl in contact with the materials;

said impeller having a pair of blades radially extending outwardly from a central post;

an upper and a lower impeller section coaxial with respect to each other and arranged in fixed spaced relationship and extending in planes diagonally disposed With respect to each other;

said pair of blades formed with the ends of said impeller sections so that said blades drive the material downwardly in the direction of rotation;

a lid for said bowl having an annular inwardly sloping periphery adapted to sealingly engage with the interior wall surface of the bowl and an undersurface spaced by said blades to provide a substantial gap therebetween; and

power means for selectively rotating said impeller to thoroughly mix the material held in the bowl and to stir the material so as to effectively remove the presence of entrapped air in the material.

2. The invention as defined in claim 1 wherein each of said blades includes a material driving face arranged at an angle within the range of 12-16 to the material being driven and wherein each blade is formed with an opening permitting the passage of a limited amount of material therethrough.

3. The invention as defined in claim 2 wherein said blade openings are co-extensive to provide a single enlarged opening having a configuration substantially the same as the general external configuration of said pair of blades.

4. The invention as defined in claim 2 wherein said blade opening includes a circular passage formed in each blade and being separated from each other by said central post.

5. The invention as defined in claim 1 wherein.

the dental material being mixed is a special dental preparation for making a dental mold composed of powder and water whereby the downwardly driven material commingles said powder and said water to form a dental composition known as alginate.

6. The invention as defined in claim 1 wherein the dental material being mixed is a special dental preparation for making a dental cast composed of plaster of Paris powder and water whereby the downwardly driven material cornmingles said powder and water to form a dental composition known as stone.

7. The invention as defined in claim 1 wherein the material being mixed may be a special dental preparation known as alginate or stone.

8. Apparatus for stirring and mixing dental preparations known as alginate and stone in a flexible bowl comprising:

a high speed impeller having a central post coaxially disposed in the bowl in contact with the preparation;

said impeller having a pair of co-extensive blades located apart and angularly disposed with respect to said central post, each blade having a face disposed at an angle within the range of 12-16 to the forward direction of impeller rotation adapted to urge the preparation downwardly against the interior wall surface of the bowl, each blade of said pair being formed with a circular opening and being laterally separated from each other by said central post in fixed spaced relationship;

a lid rotatably supporting said impeller thereon and having an annular inwardly sloping periphery adapted to sealingly engage with the upper portion of the interior wall surface of the bowl to obviate the flow of the preparation therebetween; and

power means for selectively imparting rotary movement to said impeller to thoroughly mix the preparation at high speed in a short period of time.

9 10 9. The invention as defined in claim 8 wherein References Cited said lid extends into the interior of the howl so that its UNITED STATES PATENTS undersurface terminates in close proxnnity to the top of said pair of impeller blades so as to inhibit the 640,369 1/1900 cross 259-134 flow of the preparation therebetween during mixing. 5 1:665:26O 4/1928 Grunn 259-1O7 10. The invention as defined in claim 8 wherein 1,751,097 1930 N ls n et a 259107 said lid extends into the interior of the bowl so that 1,839,082 12/1931 Burgard 259-108 a substantial gap exists between its undersurface and 1,863,977 6/1932 Forde et a1 259-108 X the p of said p of impeller blades so as to P 2,696,022 12/1954 Steinbock et a1. 259-s X mit the flow of the preparation therebetween during 10 IniXiIlg- WILLIAM I. PRICE, Primary Examiner. 

1. APPARATUS FOR STIRRING AND MIXING DENTAL MATERIALS SUCH AS STONE IN A FLEXIBLE BOWL FOR HOLDING THE MATERIALS COMPRISING: A HIGH SPEED IMPELLER COAXIALLY DISPOSED WITHIN THE BOWL IN CONTACT WITH THE MATERIALS; SAID IMPELLER HAVING A PAIR OF BLADES RADIALLY EXTENDING OUTWARDLY FROM A CENTRAL POST; AN UPPER AND A LOWER IMPELLER SECTION COAXIAL WITH RESPECT TO EACH OTHER AND ARRANGED IN FIXED SPACED RELATIONSHIP AND EXTENDING IN PLANES DIAGONALLY DISPOSED WITH RESPECT TO EACH OTHER; SAID PAIR OF BLADES FORMED WITH THE ENDS OF SAID IMPELLER SECTIONS SO THAT SAID BLADES DRIVE THE MATERIAL DOWNWARDLY IN THE DIRECTION OF ROTATION; A LID FOR SAID BOWL HAVING AN ANNULAR INWARDLY SLOPING PERIPHERY ADAPTED TO SEALINGLY ENGAGE WITH THE INTERIOR WALL SURFACE OF THE BOWL AND AN UNDERSURFACE SPACED BY SAID BLADES TO PROVIDE A SUBSTANTIAL GAP THEREBETWEEN; AND POWER MEANS FOR SELECTIVELY ROTATING SAID IMPELLER TO THOROUGHLY MIX THE MATERIAL HELD IN THE BOWL AND TO STIR THE MATERIAL SO AS TO EFFECTIVELY REMOVE THE PRESENCE OF ENTRAPPED AIR IN THE MATERIAL. 